Synchrotron radiation is essential for our studies on a variety of proteins that function either in the assembly of HIV virus particles or in the regulation of intracellular proteolysis. Data collected at SSRL during 1995 allowed us to determine the structure of the matrix protein from HIV-1. This protein directs assembly of virus particles by binding to the cell membrane. The surprising observation that matrix forms a trimeric assembly explains a number of biological phenomena and suggests a model for how matrix binds to membranes.